The present invention relates to a magnetically anisotropic rare earth-based permanent magnet material and a method for the preparation thereof.
In the modern magnet industry, rare earth-based permanent magnets are under mass production including those based on a samarium-cobalt alloy and those based on a neodymium-iron-boron alloy, of which the demand for the neodymium-iron-boron permanent magnets, referred to as the Nd--Fe--B magnets hereinafter, is rapidly expanding by virtue of their high magnetic properties and inexpensiveness due to the low material costs as compared with the samarium-cobalt magnets.
Several methods have been developed for the manufacture of the Nd--Fe--B magnets and are now industrially practiced, of which the most widely employed is the so-called sintering method. An Nd--Fe--B magnet prepared by the sintering method has a metallographic structure consisting of a magnetically hard phase of Nd.sub.2 Fe.sub.14 B as the principal phase in combination with other phases including a phase rich in the content of Nd and a phase rich in the content of B such as Nd.sub.1.1 Fe.sub.4 B.sub.4. In the preparation of such an Nd--Fe--B magnet, an alloy of Nd, Fe and B having a chemical composition somewhat richer in the conents of Nd and B than the stoichiometric composition of Nd.sub.2 Fe.sub.14 B is finely pulverized into a powder having a particle size of a few micrometers and the fine powder is compression-molded into a powder compact in a magnetic field to align the easy magnetization axes of the particles along the direction of the magnetic field. The thus prepared green body is subjected to a sintering treatment at a temperature of about 1100.degree. C. followed by an aging treatment at a lower temperature to complete a magnetically anisotropic permanent magnet material (see, for example, M. Sagawa, et al. Japanese Journal of Applied Physics, volume 26, page 785, 1987). The mechanism for the appearance of the high coercive force in the anisotropic permanent magnet of this type is presumably that the principal phase of Nd.sub.2 Fe.sub.14 B has the interfacial surface cleaned by the Nd-rich phase surrounding the same.
An alternative method for the preparation of the Nd--Fe--B magnets has been developed, in which the base material of the magnet is a quenched thin ribbon of the alloy obtained by the so-called melt-spun method to effect high-speed solidification of the alloy melt ejected at a rotating cooling roller (see, for example, R. W. Lee, Physics Letter, volume 46, page 790, 1985, and elsewhere). Although the magnet prepared by this method also has a metallographic structure of Nd.sub.2 Fe.sub.14 B as the principal phase, the high coercive force of the magnet is a consequence of the extremely small crystal size as compared with the sintered magnets ranging from 20 to 100 nm to be comparable with the size of the single magnetic domains.
On the other hand, extensive investigations are now under way to develop a further upgraded rare earth-based permanent magnet material for the next generation and, as a result thereof, so-called nanocomposite magnets are highlighted in recent years (see, for example, E. F. Kneller, et al., IEEE Transaction Magnetics, 1991, page 3588, and elsewhere). A nanocomposite permanent magnet has a structure consisting of a magnetically hard phase and a magnetically soft phase each finely dispersed in the other in a fineness in the order of a few nanometers. In this structure, the magnetization of the magnetically soft phase is little susceptible to reversal as a consequence of magnetic coupling of the exchange interaction between the two phases so that the magnetic behavior of the magnet resembles that of a single magnetically hard phase. Namely, a possibility is given by the technology of nanocomposite magnets for accomplishing a greatly increased saturation magnetization without any decrease in the coercive force even by starting from a combination of conventional base materials. For example, a theoretically possible highest value of the maximum energy product (BH).sub.max of as high as 137 MGOe is reported by calculation for a rare earth-based magnetically anisotropic nanocomposite a magnet having a composition of a combination of the magnetically hard and soft phases expressed by the formula Sm.sub.2 Fe.sub.17 N.sub.3 /Fe--Co (see R. Skomski, et al., Physical Review B, volume 48, page 15812, 1993).
Several rare earth-based nanocomposite permanent magnets have heretofore been actually prepared for the combinations of the hard and soft phases including those expressed by the formulas Nd.sub.2 Fe.sub.14 B/Fe.sub.3 B (R. Coehorn, et al., Journal de Physique, volume 49, page C8-669, 1988), Nd.sub.2 Fe.sub.14 B/Fe (Japanese Patent Kokai 7-173501 and 7-176417; L. Withanawasam, et al., Journal of Applied Physiques, volume 76, page 7065, 1994, and elsewhere) and Sm.sub.2 Fe.sub.14 B/Fe (J. Ding, et al., Journal of Magnetics and Magnetic Materials, volume 124, page L1, 1993). In these experimental works, an amorphous alloy is first prepared by the roller quenching method or by the mechanical alloying method followed by a heat treatment of the alloy to effect microcrystallization for a finely dispersed structure or, alternatively, by conducting the roller quenching method at a somewhat decreased quenching rate to effect in situ microcrystallization. In each of these methods, the process conditions are optimized so as to accomplish possible highest values of the magnetic properties such as the coercive force and residual magnetization.
Since the crystallite diameter in these nanocomposite magnet materials is so fine as to be a few tens of nanometers, a powder of a particle diameter of several micrometers obtained by merely pulverizing the material consists of particles each containing a large number of magnetically hard crystalline grains such as Nd.sub.2 Fe.sub.14 B and Sm.sub.2 Fe.sub.17 N.sub.3 and magnetically soft crystallite grains such as Fe and Fe.sub.3 B in a random orientation so that the alloy particles per se are each isotropic. As a consequence, the particles cannot be aligned relative to the easy magnetization axes of the magnetic particles as in the anisotropic sintered magnet so that the magnetic properties of the magnets obtained by these methods are necessarily inferior.
On the other hand, it is known that the reaction between Fe and Nd.sub.2 Fe.sub.14 B is peritectic (see M. Sagawa, Japanese Journal of Applied Physics, volume 26, page 785, 1987) so that, when an alloy melt having a composition richer in the content of Fe than the stoichiometric composition of Nd.sub.2 Fe.sub.14 B is cooled down and solidified in a conventional manner as in the preparation of the sintered magnets, incipient crystals of Fe are first formed to be followed by the solidification and formation of the Nd.sub.2 Fe.sub.14 B crystals. The incipient crystals of Fe are each dendritic having a size of at least several micrometers to be oversize for the above mentioned magnetic coupling. This results not only in the failure of obtaining a nanocomposite structure of the magnet but also, if such coarse dendritic crystals remain in the final magnet structure, in a great decrease in the magnetic properties of the magnet because the demagnetization curve of the magnet exhibits a cornered bent in the vicinity of zero-magnetic field.
Thus, despite the so great possibility of obtaining a high-grade permanent magnet by the theoretical consideration, no reports are available for the actual preparation of a rare earth-based nanocomposite permanent magnet, development of which is eagerly desired in the industry.